The shield you're referring to is known as a Roman scutum. Your best bet in creating it is to get a
large blue barrel (you can find these used at some recycling shops for, like, $6), and cut off the top and bottom, and then cut it in half, or slightly more if you want the curve less extreme. After that, get either a good sturdy trowel handle, or part of a 2x4, and shape it to fit the curve of the inside of the shield; if using a 2x4, be sure to carve it into a handle first, and leave space for bolts. Place this handle horizontally (for historical accuracy and awesome comfort) about 1/4 of the way from the top of the shield, and bolt it in place. For the bolting, I recommend using tee-nuts. They'll be a bit more difficult to use on a curved plastic, but you otherwise will have to tear off the face of the shield when you have to replace the handle, or when the bolts come loose, as the heads of the bolts on the foam side will have nothing to grab onto, and will simply turn in place. Next, put two layers of foam on the face of the shield, add two layers to the edge, add a final layer to the face, and add another two layers the edge. This will give you a safe, awesome shield that you can use aggressively. It will be a bit heavy, but I've handled one built in this manner before (it was owned by a player with a gladiator persona), and it was pretty sweet.
Now, to get it to look exactly like that shield is going to require some plasti-dipping. Plasti-dip is a liquid rubber/foam/thing that is very popular in the boffer world for it's ability to safely and uniformly cover artistic detail on non-striking surfaces, and because it is a great paintable surface, while still remaining soft enough for incidental padding, such as shields, haft padding, and pommels. Honestly, there's more to it than I can even hope to explain in a short paragraph, so instead, go check out these links:
https://www.facebook.com/groups/plastidip/http://www.calistonarmory.com/And, of course, for more information on the more specific methods of shield construction, if you are unfamiliar with them, check out the
Geddon. For a project like this, I would recommend holding off on the plastidipping and just going with a cloth cover until you are more comfortable as a foamsmith. PDing can be a very expensive project, and is very unforgiving of mistakes.
Cheers,
Thurat
P.S. An alternative to using the blue barrel that I had forgotten about, would be
an all foam version. You'll also find a tutorial on the Geddon for a shield as I have described above, but using metal bracing. The shield i handled did not have this. However, that design also has the handle placed into a hole cut into the plastic, which created a weak spot, so that author may have found that necessary. If you do the handle as I described, you should be fine. If you do it as done in that
other tutorial, then follow those directions and use the bracing.